Ecology: Lecture 13
Interspecific Competition 2
November 5, 2007
Definition of a niche:
Theodore Giesel
And NUH is the letter I use to spell Nutches,
Who live in small caves, known as Niches, for hutches.
These Nutches have troubles, the biggest of which is
The fact there are many more Nutches than Niches.
Each Nutch in a Niche knows that some other Nutch
Would like to move into his Niche very much.
So each Nutch in a Niche has to watch that small Niche
Or Nutches who haven't got Niches will snitch.
-On Beyond Zebra (1955)
Definition of a niche
(from Hutchinson and Shelford)
 For each environmental variable it
faces in its world, an organism has a
range of values for which it can grow
and multiply
A space of n-dimensions
Defining and mapping niches
(based on key characteristics)
Niche relationships among species
x-axis: state of a particular resource (size of
prey, for example)
y-axis: species response/fitness (much as for
Shelford’s law of tolerance)
Niche overlap:
mapping >1 variable
Fundamental vs. realized niche
Competitive exclusion principle

Gause: “As a result of competition, two
similar species scarcely ever occupy
similar niches, but displace each other
in such a manner that each takes
possession of certainly particular kinds
of food and modes of life in which it
has an advantage over its competitor.”

Hardin: “Complete competitors cannot
coexist.”
Resource partitioning
Resource partitioning:
Mojave desert plants
Resource partitioning:
Christmas Island terns
Other responses to competition (spatial
rather than resource-based)
 Niche compression
 Example: Doves in New Guinea
 Niche shift
 Example: Sunfish in artificial ponds
 Ecological release
Character displacement:
Darwin’s finches
So, how is it possible for so many similar
species of tropical fish to exist?
So, how is it possible for so many
similar species of tropical fish to exist?

Explanations consistent with the
competitive exclusion principle

i.e. resource partitioning, niche compression,
niche shift


Essentially, these suggest extreme specialization
Multiple niche dimensions must be
considered (see. Fig. 14.21)

What might some of these “dimensions” be?
So, how is it possible for so many
similar species of tropical fish to exist?
Non-equilibrium conditions


Factor 1: Transient nature of larvae



Factor 2: Environmental disturbance (i.e. storms)



Cause mortality
How could this prevent competitive exclusion?
Factor 3: Predation!


What does this mean?
How could this prevent competitive exclusion?
Predators may target the species that are most abundant,
so that less abundant species are able to increase.
How can we relate this back to the LotkaVolterra competition equations?